Covering materials for electric wires are desired to have a reduced permittivity so as to improve the electrical characteristics. Such reduction in permittivity can effectively be achieved by using foams as covering materials. Foams made of resin material can usually be produced by a foam-molding process in which gas is introduced into a molten resin and then molded.
In order to equalize the shape and the properties of the resulting foam, the cells in the foam are preferably fine and uniformly distributed therein. For the purpose of making the cells fine and of distributing the cells uniformly, a foam nucleating agent, which is to serve as the starting point of generating cells during the foam molding, may be introduced into a resin.
One typical example of the foam nucleating agent added to resin such as fluororesin is boron nitride (BN) because BN has thermal stability, chemical inertness, low toxicity, and excellent electric properties (for example, it causes only a slight change in permittivity when added).
The following disclosures provide examples of a method of producing a foam by mixing BN as a foam nucleating agent to resin such as fluororesin.
Patent Literature 1 discloses that a foam nucleating agent is preferably a combination of BN with zeolite among combinations of any of ceramics and materials thereof with zeolite.
Patent Literature documents 2 and 3 disclose a method in which BN is used as a foam nucleating agent.
Patent Literature 3 also discloses a method in which BN having a surface area of 5 to 10 m2/g is added as a foam nucleating agent.
Patent Literature documents 4, 5, and 6 disclose combination use of boron nitride having a surface area of 8.6 m2/g with any of sulfonic acids, phosphonic acids, and salts thereof.
Patent Literature documents 7 and 8 disclose combination use of BN having a surface area of about 8 m2/g with any thermally stable inorganic salt which comprises metal cations and polyatomic anions and which satisfies a specific formula, such as sodium tetraborate.
Patent Literature 9 discloses that BN used in the formation of fluororesin foams preferably has an average particle size of smaller than 10 μm and a resin and the BN are pulverized using an appropriate mill.
Patent Literature 10 discloses a method in which a resin powder foamable by decarboxylation, BN, and a metal salt are mixed to provide a blend, and next the blend is coarsely pulverized into a pulverized powder using a compressor and then a grinder such as a hammer mill, or the blend is formed into pellets using a pellet mill, and thereafter the powder or the pellets are fed into an extruder. The literature discloses that the mixing is preferably performed with a HENSCHEL mixer (Purnell International).
Patent Literature 11 discloses a foaming method in which a processible fluoropolymer material and BN, as well as a sulfonic acid, a phosphonic acid, or a salt thereof, and/or a polyatomic anion-containing inorganic salt are used together, wherein the BN crystals are grown to the final size and have an average particle size of 12 μm or smaller. The literature discloses that, when the respective grown crystallites agglomerate, the BN crystals are prepared not by mechanical pulverization, in which the respective crystals are pulverized, but by deagglomeration.
Patent Literature 12 discloses a resin composition containing a fluororesin (A) and a foam nucleating agent (B), wherein the foam nucleating agent (B) has a d99 value of 15 μm or smaller. Boron nitride is used as the foam nucleating agent (B).
Patent Literature 13 discloses a composition containing a specific fluorine-containing copolymer suited for use in the fabrication of foamed articles. The literature also discloses that the composition may contain boron nitride. Patent Literature documents 14 and 15 disclose a fluorine-containing polymer which has a high melt flow rate and which is capable of being extruded at high speed, and also disclose boron nitride as a foam nucleating agent to be added to the fluorine-containing polymer.
Patent Literature documents 16 and 17 disclose a fluororesin composition comprising 100 parts by mass of a tetrafluoroethylene/hexafluoropropylene copolymer and 0.01 to 3 parts by mass of polytetrafluoroethylene having a standard specific gravity of 2.15 to 2.30, wherein the fluororesin composition is obtainable by mixing an aqueous dispersion of the tetrafluoroethylene/hexafluoropropylene copolymer and an aqueous dispersion of the polytetrafluoroethylene, coagulating the dispersed matter, and then drying the coagulum and melt-extruding the dried matter. The documents also disclose that a foamed electric wire is produced from the composition containing boron nitride.